1. Field of the Invention
The present invention relates to capacitance detection circuits and, more particularly, to a capacitance detection circuit that detects minute changes in capacitance.
2. Description of the Related Art
A variable capacitor is a well-known circuit element whose capacitance, as the name implies, can vary. Variable capacitors are commonly used as a point of control where a control signal is used to set or change the capacitance. For example, a control signal can be used to set or change the capacitance of a variable capacitor in a LC circuit which, in turn, sets or changes the resonant frequency generated by the LC circuit.
Variable capacitors are also commonly used as a sensor where the change in capacitance occurs as a result of a change in an external physical condition. For example, a variable capacitor can be formed in a micro-electromechanical system (MEMS) integrated circuit where the change in capacitance occurs as a result of a change in acceleration, pressure, or some other external physical condition.
A MEMS integrated circuit is a semiconductor structure that is fabricated using the same types of steps (e.g., the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital CMOS circuits. MEMS integrated circuits differ from conventional integrated circuits in that MEMS integrated circuits include movable micro-structures.
One example of a movable micro-structure is a thin plate structure that is formed over a cavity where the thin plate structure physically moves in and out of the cavity when the MEMS integrated circuit is subjected to an external physical condition. For example, when the MEMS integrated circuit (which is connected to some other body) accelerates, the thin plate structure moves in response to the acceleration.
The thin plate structure and cavity can be utilized to form a variable capacitor by forming a first electrode on the thin plate structure and a second electrode on, for example, the bottom of the cavity. In operation, the capacitance of the MEMS variable capacitor changes as the plate structure moves and the distance between the first and second electrodes of the capacitor changes.
In a MEMS integrated circuit, both surfaces of a plate structure can be used to support electrodes. For example, the first electrode of one capacitor can be formed on the top surface of the plate structure, while the first electrode of another capacitor can be formed on the bottom surface of the plate structure.
Regardless of how a variable capacitor is used, it can be difficult to detect the change in capacitance. When used as a point of control, it can be difficult to determine whether the control signal actually produced the desired capacitance or change in capacitance. Age, temperature, and other factors can cause the actual capacitance to differ from the desired capacitance.
When used as a sensor, it can be difficult to detect minute changes in capacitance and, thereby, minute changes in the external physical condition. Thus, there is a need for an approach to detecting changes in capacitance.